X-ray study of the low-temperature phase transition in K2CoCl4

Precise lattice parameter measurements and intensity measurements of selected main and satellite reflections of K₂CoCl₄ have been performed in the temperature range 100 to 300 K in the vicinity of the low-temperature phase transition (commensurate-commensurate phase transition, T _c = 142 K). A broadening of the FWHM for the h01 reflections was observed below 142 K which suggests a transition from an orthorhombic phase to a monoclinic phase.     

X-ray study of the superionic phase transition in CuCrS2

The temperature disorder of copper ions in the CuCrS₂ superionic conductor was studied by X-ray analysis. It is shown that the average square deviations of atoms sharply increase and become comparable with the distance between neighbouring tetrahedral sites when CuCrS₂ transforms into the superionic phase. It is also established that the direction of maximum deviation of atoms is close to the direction connecting the tetrahedral positions.     

(1)H and (87)Rb nuclear magnetic resonance study of the order-disorder phase transition of RbHSeO(4) single crystals

The ferroelectric phase transition at T(C2) (=370K) in RbHSeO(4) has been studied by (1)H and (87)Rb solid-state NMR. Although not large, the spin-lattice relaxation time, T(1), and the spin-spin relaxation time, T(2), of rubidium and of the alpha- and beta-type protons show distinct change near the phase transition. The intensity of the signal due to the alpha-type protons decreases with increasing temperature, and the intensity of alpha-type protons is quite weak above 330K: at a temperature which is about 40K lower than the phase transition temperature, the ordering of the alpha-type protons occurs. The alpha-type protons in the ferroelectric phase lead to a noticeable change in the proton magnetic resonance spectra. Our study of the (1)H spectra shows that the ferroelectric phase transition in RbHSeO(4) is of order-disorder type and is due to the ordering of protons in hydrogen bonds.     

X-ray study of the PZT solid solutions near the morphotropic phase transition

In the solid solutions Pb(Zr_xTi_1?x)O₃ it is stated that for x ? 0.55 there is a morphotropic phase change. Thermodynamically, it is expected that there will be a region of x, in which the two phases coexist and obey the leverage law. By X-ray diffraction, we found that the two phases coexist for x between 0.49 and 0.64. In this range the lattice parameters do not change, and the leverage law is obeyed. We conclude that an adequate explanation of the particular properties of the PZT solutions has to be found.     

X-ray diffraction study of phase transition of ferroelectric liquid crystal DOBAMBC

Cu₆PS₅Br superionic crystals were implanted by sulphur ions. The effect of ion implantation on the phase transition (PT) temperatures in Cu₆PS₅Br is observed from isoabsorption studies of the optical absorption edge. The dependences of the ferroelastic PT temperature on the ion fluence are analysed.     

Acoustic study of the ferroelastic phase transition in LiCsSO4 crystal

The paper reports on an acoustic study of the temperature dependences of the ultrasonic-wave velocity and attenuation in a LiCsSO₄ crystal within the 190–295 K temperature region, which includes the interval of the pseudoproper second-order ferroelastic phase transition (202 K). The velocity of the transverse xy acoustic mode is found to decrease by more than six times at the phase transition. The possibility of performing ultrasonic studies both in the region of the ferroelastic phase transition temperature and below it is demonstrated. The results are treated in terms of Landau’s theory. Waves not associated with the soft mode are shown to exhibit anomalies which are supposedly due to an intermediate phase, whose existence was suggested in a number of publications.     

X-ray structural investigations of the phase transition in benzyl crystals

The Bragg (fundamental) and diffuse reflections from the M-points of the Brillouin zone boundary of the paraphase of the benzyl crystals, whose integrated intensity is related to the order parameter and its fluctuations, respectively, have been investigated. The original information on the temperature behavior of the order parameter and the diffuse scattering is obtained and the values of the corresponding critical exponents are determined. The temperature behavior of the correlation radius and its anisotropy are analyzed. The conclusion is drawn that the phase transition in these crystals is close to the tricritical point. The mechanism of the phase transition in the benzyl crystals alternative to the “trigger” mechanism is suggested.     

Performance of different types of detectors in a high-pressure X-ray study of phase transition

Abstract

Phase transitions in praseodymium and lanthanum under pressure have been studied using a synchrotron powder X-ray diffraction technique. A structure refinement of the distorted fcc phase of Pr using diffraction data collected with an imaging plate (IP) detector demonstrate that among some possible structures the rhombohedral structure with space group R3m best reproduces the observed diffraction pattern. The distorted fcc-fcc phase transition in La was observed as a function of the temperature at 23 GPa using a CCD-based detector. A five-minute exposure sufficiently long to measure the intensities of very weak superlattice reflections from the distorted fcc phase, which has been found to transform to the fcc phase at 550 K. The performance of the IP and a CCD-based detector are compared and their future developments discussed.     

EPR study of the ferroelastic phase transition in CsLiSO4

EPR spectra of SO^-₃ ion-radical in X-ray irradiated CsLiSO₄ single crystals were used for the study of the ferroelastic phase transition at T_c = 203.0 K. The splitting (ΔH) of the SO₃ line in the low-temperature ferroelastic phase has been interpreted as proportional to the square of the order parameter. The splitting shows the temperature dependence ΔH ∞ (T_c ? T) 1.01^± 0.01     

Dielectric study of the ferroelectric phase transition of KH2PO4

We report high precision measurements of the dielectric constant of Potassium Dihydrogen Phosphate as a function of temperature and electric field. Landau model parameters are evaluated and the biasing field E_c which drives the transition second order is determined.     

Intermediate-temperature polymorphic phase transition in KH2PO4: A synchrotron X-ray diffraction study

We have used synchrotron X-ray diffraction to investigate the structural and chemical changes undergone by polycrystalline KH₂PO₄ (KDP) upon heating within the 30-250 °C temperature interval. Our data show evidence of a polymorphic transition at T∼190 °C from the room-temperature tetragonal KDP phase to a new intermediate-temperature monoclinic KDP modification (spacegroup P2₁/m and lattice parameters a=7.590, b=6.209, c=4.530 Å, and β=107.36°). The monoclinic RDP polymorph remains stable upon further heating to 235 °C, and is isomorphic to its RbH₂PO₄ and CsH₂PO₄ counterparts.     

X-ray diffraction study of a structural phase transition in (NbSe4)3I

The second-order structural phase transition was observed by X-ray diffraction technique in (NbSe₄)₃I. The transition temperature (T_c) was 274.2 K. No evidence of the formation of the charge-density wave was found out. Temperature dependence of the intensity of (0,5,12) reflection obeys the Landau theory of the second kind of phase transition near T_c. Temperature dependence of the integral breadth of (0,0,12) reflection indicates the effect of the soft-phonon mode below T_c. The existence of the structural phase transition, probably not associated with charge-density-wave formation, suggests the unique nature of (NbSe₄)Z₃I in the new series of compounds (MSe₄)_xI (M=Nb, Ta).     

Powder neutron diffraction study of low temperature phase transition in LiKSO4

The salient features of the neutron diffraction measurements on polycrystalline sample of LiKSO₄ at 170 K and 104 K are reported. The sample undergoes a phase transformation to a single phase as against “mixed phases” reported in single crystal studies. These differences are discussed on the basis of the kinetics of nucleation and growth processes.     

High-pressure phase transition in CaTiOSiO4 titanite

The P2₁/a to A2/a transition in titanite has been studied at high pressure (and room temperature) by X-ray powder diffraction. On the basis of the disappearance of k + l= odd reflections from the diffraction pattern, the transition was located between 3·351(3) and 3·587(3) GPa. The variation with pressure of the spontaneous strain in the P2₁/a phase indicates that the transition has an effective critical exponent significantly less than 1/2. Within the uncertainties of the data, the transition could be weakly first-order in character or be continuous with an effective critical exponent in the range of ～0·13-0·25. The A2/a to P2₁/a transition is accompanied by a significant expansion of the a-axis as a result of the bond-valence sum requirements of the Ti atoms: in the high-pressure phase they occupy the centres of the TiO₆ octahedra, but they are displaced along the a-axis in the P2₁/a phase to form alternating short and long Ti-O bonds.     

Calorimetric and single crystal X-ray study of the phase transition of (PyH)2PdCl4

Polymorphic transition of pyridinium tetrachloropalladate(II) was investigated by heat capacity measurements and by single crystal X-ray structural analysis. A large λ-type anomaly was detected at 240 K in the temperature dependence of the heat capacity. The low-temperature phase (LTP) belongs to the triclinic space group with a=6.856(1), b=7.293(1), c=7.721(1) Å, α=75.180(2)°, β=71.081(2)°, γ=81.109(3)° at 100 K, and the high-temperature phase (HTP) to the same space group with a=7.217(2), b=7.470(2), c=7.880(2) Å, α=73.438(3)°, β=65.195(3)°, γ=82.727(4)° at 293 K. The pyridinium cations are ordered antiferroelectrically in LTP. In HTP, however, an orientational disorder of the cation was observed. The energy difference between potential wells for the reorientation of pyridinium ion in HTP is discussed referring to the results of the present single crystal X-ray and heat capacity as well as the previous ¹H NMR measurements. A five-site disorder model is shown to be consistent with both of the observations of ¹H NMR and X-ray study.